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Full text of "Text Book Of Mechanical Engineering"

884             m                Appendix II.

Pf 614. Calculation of Initial Condensation and
Leakage.  The weight of steam used in addition to that shewn
by the indicator is a loss spoken of as the ' missing quantity/
and is ascribed to cylinder condensation and leakage. The
researches of Him, D. K. Clark Willans, Cotterill, Thurston,
Donkin, Adams, English, and Callendar have been directed
towards the rationalisation of this loss, in order to predict it
in any case where the conditions of pressure, speed, clearance,
expansion, &c., are known. Donkin, Adams, and Callendar have
experimentally obtained temperature cycles of the steam and the
cylinder walls, which fully shew the interchanges that have been
generally admitted. They also shew that steam temperatures
may be accurately deduced from the indicator diagram by con-
verting pressures into corresponding temperatures. Cotterill
devised a semi-empirical formula, based on barrel surface, as
follows : 

(C loge r\
i + --- 7=v~ I
d ^/N /

where d = cylinder diameter. Calendar's experiments appear to
shew the greatest loss to be due to clearance surface, while
leakage may be -responsible for from 5 to 50% of the total loss.
The constant G is about 6 to 8 in simple unjacketed engines,
and 4 in jacketed slide-valve engines, 2 to 4 in Corliss engines
depending on the presence or absence of jackets, and 12 in very
bad engines ; but rarely rises above 8. Cotterill calls the
bracketed terms 'liquefaction factor,' and another method of
statement is

Missing steam  __ C loge r
I ndicated steam

The missing quantity may vary from 25 to 60% of total steam
supplied, and is about 30% on the average. Fig. 755 shews the
loss graphically. ( See /. 1134.)

jP. 614. Methods of Reducing Condensation.  There
are four methods of decreasing the loss just mentioned : quick
ranning, steam-jacketing, superheating, and compounding.

Quick running.  The advantage of high rotational speed (not